The average arrival rate is expressed in units of work and units of time. In the prior time card example, the arrival rate is likely to be expressed in terms of transactions and minutes. The math involved is very straightforward: divide the total number of transactions that arrived by the time interval. For the example, it would be 9,000 transactions divided by 30 minutes, which is 300 trx/min.

There can be a difference between the rate of transaction arrivals (or entry) and the rate of transaction exits. The actual transactions being processed is known as the workload. A system is deemed stable when, on average, the transaction entries equal the transaction exits. If this does not occur, eventually either so many transactions will build up on the system that it will physically shut down, or there will be so few transactions that no work will be performed. Because of this equality, for our work with Oracle systems, it is acceptable to refer to the arrival rate as the workload and vice versa. Use the term that makes your work easily understandable for your audience.

The arrival rate symbol is universal in all publications and it is the Greek letter lambda. For the example of an arrival rate (the work performed over a period of time) of 9,000 transactions over a 30-minute period, using symbols and converting to seconds, the arrival rate calculation is as follows: